1. Field of the Invention
This invention relates to a process for the production of light-colored .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes, in which the crude sulfonation product is subjected immediately after the sulfonation reaction to a temperature-controlled after-reaction in a substantially ideally mixed liquid phase until a degree of sulfonation of at least 90% is reached.
2. Statement of Related Art
.alpha.-Sulfofatty acid alkyl ester alkali metal salts are acquiring increasing significance as surfactants for detergents and cleaning preparations based on renewable natural raw materials. In known processes, the .alpha.-sulfofatty acid alkyl ester alkali metal salts are obtained in the form of aqueous solutions or pastes by neutralization of .alpha.-sulfofatty acid alkyl esters which may be synthesized by reaction of lower fatty acid alkyl esters with gaseous SO.sub.3. In the final analysis, the basis for the production of the .alpha.-sulfofatty acid alkyl ester alkali metal salts are fats and oils of natural origin from which the lower fatty acid alkyl esters are obtained by lipolysis and subsequent esterification of the free fatty acids with lower alkanols or by transesterification of the natural triglycerides with lower alkanols. In both reactions, methanol is preferably used as the lower alkanol. The lower fatty acid alkyl esters are mixtures in which C.sub.6-22 fatty acid residues occur, the chain length distribution being dependent on the origin of the natural fats or oils. In many cases, these fatty acid ester mixtures are not used for the synthesis as such, but rather in the form of certain fractions. Sulfonation of the fatty acid ester mixtures with gaseous SO.sub.3 gives acidic .alpha.-sulfofatty acid alkyl esters which are converted into aqueous pastes of .alpha.-sulfofatty acid alkyl ester alkali metal salts by neutralization to a pH value of 6 to 8. The crude .alpha.-sulfofatty acid alkyl esters and their alkali metal salts are more or less colored products which generally have to be treated with typical bleaches before and/or after neutralization.
The sulfonation of the fatty acid alkyl esters is normally carried out with gaseous SO.sub.3 at temperatures in the range from 30.degree. to 100.degree. C., the molar ratio of fatty acid ester to SO.sub.3 being from 1:1.2 to 1:1.8. The reaction of the fatty acid alkyl esters with SO.sub.3 takes place in two steps. In a first, rapid step, 2 molecules SO.sub.3 react with 1 molecule fatty acid alkyl ester to form a mixed anhydride of .alpha.-sulfofatty acid and alkyl sulfuric acid. In a slow, second step, the mixed anhydride acts as sulfonating agent for unreacted fatty acid alkyl ester. The first step takes place in the particular typical sulfonation reactor used, for example in a falling-film reactor or in a sulfonation cascade. Very little information is available to show how the second reaction step is carried out in known processes, generally being confined to the reaction temperature and, at most, to the reaction time. Thus, according to DE-OS 31 23 681, the sulfonation product is said to be aged at 30.degree. to 100.degree. C. According to DE-OS 33 34 517, the sulfonation product is aged for 10 to 20 minutes at 80.degree. to 100.degree. C.
During the work culminating in the present invention, it was found that the conduct of the second step of the sulfonation reaction is crucially important to the degree of sulfonation and to the quality of the end product. It has been found that product quality depends to a large extent on exact temperature control and on a narrow holding time distribution in the after-reaction. Short holding times lead to readily bleachable, but inadequately sulfonated .alpha.-sulfofatty acid alkyl ester alkali metal salt pastes. Long holding times lead to high degrees of sulfonation, but also to strongly colored products which cannot be bleached to the necessary color values after neutralization.
Known holding-time installations are unable to satisfy the dual requirement of a narrow holding time distribution and exact temperature control. Standard reactors operated solely with a heating and cooling circuit lead to far too broad a holding time spectrum on account of the constant back-mixing. Following temperature-controlled pipe coils always have to operated under full turbulent flow conditions, i.e. under plug-flow conditions, and accordingly cannot be operated independently of the output of the sulfonation reactor.
Accordingly, the problem addressed by the present invention was to provide a process which would enable the crude sulfonation product in the production of light-colored .alpha.-sulfofatty acid alkyl ester alkali metal salts by reaction of fatty acid alkyl esters with gaseous SO.sub.3 and neutralization with aqueous alkali metal hydroxide solutions to be subjected to an after-reaction under exact temperature control and, at the same time, a narrow holding time distribution before it was neutralized and bleached in known manner.